The invention relates to the use of heterocycles containing trivalent phosphorus as a ring member as catalysts for isocyanate modification and to a process for preparing polyisocyanates of the trimer type.
Polyisocyanates are generally prepared by catalysed modification of monomeric isocyanate starting materials, frequently diisocyanates. In comparison with the former they are notable for qualities including a substantially lower vapour pressure and, consequently, better physiological tolerance. Where polyisocyanates having an NCO functionality of three or more are prepared from NCO-difunctional diisocyanates, these products additionally have the advantage of higher crosslinking density in polymers generated from them, coatings for example. Particularly advantageous in this context are isocyanate trimers of the isocyanurate type and iminooxadiazinedione type (referred to collectively here, for simplification, as trimer type). Relative to the isocyanurates with the same molecular weight distribution that are based on the same monomer, iminooxadiazinediones have the advantage of a significantly lower viscosity with the same high profile of properties (cf. EP-A 798 299).
All of the prior-art catalysts previously described for the preparation of the polyisocyanates of the trimer type are hampered by the disadvantage that, following the catalysed reaction, they cannot be recovered in undecomposed form, optionally together with the unreacted fraction of the diisocyanate for modification. In general, they remain, usually in deactivated form, in the process products and/or in the unreacted starting materials (monomer), which in general are circulated. Therein they may give cause for unwanted follow-on reactions such as colour deepening, NCO drifts, etc., or have other disadvantageous effects, such as increasing contamination of the monomer which is generally circulated, for example. A further factor is that the known trimerization catalysts of the prior art are active only at a relatively high temperature, and in many cases their action sets in only after a certain ‘induction period’, which is disruptive to the process. Quite apart from this, the industrial use of relatively high-priced catalysts is prohibited by economic considerations.
The recycling of the modification catalyst without conversion—optionally, intermediary conversion—into a different form has to date been accomplished only in the case of phosphines (phosphanes) and also special pyridines, more particularly 4-dialkylaminopyridines such as, for example, 4-dimethylaminopyridine (DMAP) (DE-A 10354544, DE-A 10254878 and also J. Prakt. Chem./Chem. Ztg. 1994, 336, 185-200). Phosphines and 4-dialkylaminopyridines, however, yield predominantly polyisocyanates containing uretdione groups, which therefore have a low average NCO functionality. As a sole building block for the preparation of highly branched polyurethane polymers, especially in the paints and coatings sector, their suitability is limited.
Although according to the teaching of DE-A 1 670 720 the use of phosphines for isocyanate modification at relatively high temperature and/or with a relatively high level of monomer conversion is said to be accompanied by the formation of increasing fractions of isocyanate trimers, the products of the process nevertheless at the same time include not inconsiderable fractions of other by-products such as carbodiimides and uretonimines. Uretonimines are especially disruptive, since in the course of storage they tend to release monomeric isocyanate and the products are in that case no longer physiologically unobjectionable. Additionally, in the case of phosphine-catalysed isocyanate modification with increased temperature and/or conversion, the uretdione fraction does not decrease to the extent that it would be truly possible, in reactions in the industrially practical temperature and conversion ranges, to talk of the primary formation of products of the trimer type (Comparative Examples 1 to 4).
Very generally, phosphine-catalysed isocyanate oligomerization takes a different course when operating in the presence of aromatic isocyanates (cf. inter alia GB 1 244 416, U.S. Pat. No. 3,645,979, GB 856 372, U.S. Pat. No. 2,671,082). In this case the isocyanate ‘trimerization’ (more particularly the formation of isocyanurate) occupies the foreground significantly (Comparative Example 5). Occasionally the aforementioned patents, as well as a reference to numerous other phosphines which are said to be suitable as catalysts, in long lists, also indicate that 1-butylphosphacyclopentane (1-butylphospholane) or 1-phenyl-3-methylphosphol-3-ene might be suitable for the polymerization of isocyanates. To what extent, then, isocyanurates or iminooxadiazinediones can deliberately be prepared from purely aliphatic isocyanates by means of such catalysts, the stated patents do not reveal. This is particularly doubtful on account of the fact that trialkylphosphines specified therein, in connection with aliphatic isocyanates, are known to be highly active catalysts for the formation of uretdione, and this is in fact demonstrated with an example in GB 1 244 416, whereas in mixtures of aliphatic and aromatic isocyanates they form mixed trimers containing only a small amount of iminooxadiazinedione. Furthermore, U.S. Pat. No. 2,671,082 explicitly describes phenyl-dimethylphosphine and phenyldi(n-butyl)phosphine as preferred catalysts, which, as our own studies demonstrate, have no catalytic activity towards aliphatic isocyanates (Comparative Example 6). The same applies to 1-phenyl-3-methylphosphol-3-ene (Comparative Example 7).
Furthermore, butylphosphacyclopentane (butylphospholane), as well as other trialkylphosphines, has also been described in EP-A 1 174 428 as an optionally suitable catalyst for the dimerization of isocyanates to uretdiones. The description there is of the suitability in principle of these substances for the said purpose, the document teaching their use in combination with specific ureas or amides. There are no references in EP-A 1 174 428 to a difference in reactivity of butylphospholane to typical trialkylphosphines such as tri-n-butylphosphine. As is apparent from Comparative Examples 8 and 9, the sterically hindered phosphines tri(tert-butyl)phosphine and tri(isopropyl)phosphine, which are likewise said optionally to be suitable in EP-A 1 174 428, have no catalytic activity.
From U.S. Pat. No. 2,853,473 it is known that phospholane P-oxides can be used for the formation of carbodiimide from isocyanates. According to U.S. Pat. No. 2,853,518, the oxygen-free pendants of these P-oxides, containing trivalent phosphorus, are also said to be suitable for this purpose. The reactions described in U.S. Pat. No. 2,853,518 take place at an elevated temperature, generally without further protective measures such as an inert gas atmosphere, with generally high catalyst concentrations and also long reaction times, with direct observation of the evolution of gaseous CO2, which is characteristic of the formation of carbodiimide. Owing to the known oxidation tendency of compounds containing trivalent phosphorus, and to the absence of protective measures to ensure that in the reaction it is in fact phospholanes and not their P-oxidized analogues or other conceivable oxidation products that are present, however, it appears extremely questionable whether what was observed in the examples of U.S. Pat. No. 2,853,518 was actually the catalytic effect of phospholanes.
It has now surprisingly been found that when it has been ensured, through the presence of an inert gas atmosphere, for example, that only trivalent phosphorus is present, phospholanes and other phosphine derivatives in which the trivalent phosphorus, via two of its single bonds, is part of an organic ring system, catalyse the targeted formation of isocyanurates and iminooxadiazinediones from aliphatic and/or cycloaliphatic isocyanates, giving products which are low in uretdiones and substantially free from carbodiimides and uretonimines. Furthermore, over the whole of the target reaction range, even at low reaction temperatures, the reaction proceeds very uniformly, without an induction period and without a significant drop in activity during the catalysed reaction, and with a comparatively high fraction of iminooxadiazinedione groups in the product. Following the reaction the catalysts can be recovered, together for example with the monomer under modification, and subsequently used again.